The present invention concerns a method for industrial manufacture of a pure magnesium carbonate (magnesite) from an olivine containing species of rock while immobilizing CO2. The method allows simultaneous manufacture of other valuable products, hereunder particularly amorphous silica.
Anthropogenic emission of CO2 is by many considered to be one of the largest environmental problems in our time. The Kyoto obligations imply various types of fees when CO2 is released. There is thus a demand for cost-effective methods of handling CO2. Injection into water containing or empty petroleum reservoirs has been suggested as a method for deposition/removal of CO2 in which CO2 has been used to expel more hydrocarbons from hydrocarbon reservoirs (EOR) is a variant which may involve economical advantages.
Another possibility is to bind CO2 as carbonates by reacting CO2 with species of rock/minerals which contain elements that together with CO2 provides solid substances/minerals (carbonates) of which the most relevant elements for the manufacture are Ca, Mg, and Fe.
CO2 can e.g. be bound to MgO resulting in precipitation of MgCO3. This reaction implies that CO2 which is produced in industrial processes, like e.g. gas power plants, hydrogen production from natural gas or from methane production, can be immobilized.
Manufacture of MgO from magnesite (MgCO3) or dolomite (CaMg(CO3)2) is however not interesting since the production of MgO in these cases will imply formation of the same amount of CO2 as can be immobilized, hence the net CO2 effect is zero.
With regard to the manufacture of MgO from other sources than carbonates, like magnesium containing silicates (sulphates and phosphates) the situation is different. Such a manufacture of MgO only to immobilize CO2 will however be expensive. ON the other hand the economical potential for such a way to immobilize CO2 could be significantly improved and possibly be profitable if CO2 and water in various amounts are brought to react with a species of rock or a mineral in an industrial process where valuable products are formed during the immobilization of the CO2. An example of a suitable species of rock is dunite which mainly is comprised by the mineral olivine (+90%), which I Norway is found in large deposits at Sunnmoere and in Nordfjord. (The Aaheim occurrence alone served about 65% of the world market in 2006). This species of rock has in various acidic processes been considered as raw material for magnesium production and for production of amorphous silica.
CO2 with or without various amounts of water, used as an industrial chemical in industrial processes to provide the combined saleable products, MgCO3, amorphous silica, Fe2O3, (Fe, Mg)Cr2O4, and Pt, while simultaneously immobilizing CO2, has not previously been done.
The rock species dunite typically comprises more than 90% of the mineral olivine. Olivine occurs in a miscible (isomorphous) series in which the most common miscible minerals (end units) are forsterite Mg2SiO4 and fayalite Fe2SiO4. In the dunite occurrence in Aaheim the forsterite and fayalite components constitute 93% and 7% respectively. Olivine is not stable at the temperature and pressure conditions that exist at the earth surface. This mineral will e.g. react with CO2 in water under formation of carbonates. In nature, however, this general reaction progresses relatively slow:

The reaction rate can be increased to convenient process rates under laboratory and industrial process conditions and can be optimized by varying pressure, temperature as well as the amount ratio of CO2 to H2O.
The rock species dunite comprises in addition to the main constituent olivine minerals like e.g. chromite/magnesium chromite (Fe, Mg) Cr2O4 (0, 2-5%) and chrome containing chlorite. Chromite which is formed at deep levels in the earth crust by solidification of mafic melts or the uppermost part of the earth mantel (dunite, periodite and other ultra mafic rock species) also comprises the noble metal platinum and platinum group metals like palladium, osmium, iridium, and ruthenium. Rock species rich in chromite can contain from 0.1 to 6-7 ppm of platinum group metals. In some cases a content of up to 50 ppm has been described.
Direct carbonation of olivine has earlier been described as a one-step process (Fauth et al. 2000; O'Connor et al., 2002, 2005). The process makes use of a mixture of water, CO2 and olivine and the dissolving of olivine and the precipitation of carbonate and silica takes place in one single operation. Experiments have been performed within a temperature and CO2 pressure of 25-250° C. and 25-250 bars in closed autoclaves. Magnesium carbonate and silica are identified as reaction products. The reaction rate and degree of conversion are low especially at low temperatures and pressures. The effect of grain size, temperature and CO2 pressure has been systematically investigated. This process, however, gives little ability to distinguish between reaction products.
Multi step processes for carbonation of olivine, serpentine or other silicates have also been investigated. In these processes, however, CO2 is not included as a reactant in the first step. Blencoe et al (2004) describes a process where silicates are dissolved with lye in a first reaction chamber. The dissolved metal hydroxide is thereafter reacted with gas in a separate reaction chamber for formation of carbonates. In several processes acid is used to dissolve silicates (Maroto-Valer et al., 2005; Park et al., 2005). CO2 then is added to solution only after silicate minerals have been dissolved in an acidic solution. Dziedic et al (2004) describes a method for dissolving CO2 in water in which metal ions (Na, Mg, Ca) are added to precipitate salts from the carbonic acid. The method however does not address the problem of how to provide the metal ions.